The rotational state and ortho-para conversion of ${\mathrm{H}}_2$ on a Pd(210) surface is investigated with rotational-state-selective temperature-programmed desorption (RS-TPD) and theoretical calculations. The isotope dependence of TPD shows a higher desorption energy for ${\mathrm{D}}_2$ than that for ${\mathrm{H}}_2$, which is ascribed to the rotational and zero-point vibrational energies. The RS-TPD data show that the desorption energy of ${\mathrm{H}}_2$($J=1$) ($J$: rotational quantum number) is higher than that of ${\mathrm{H}}_2$($J=0$). This is due to the orientationally anisotropic potential confining the adsorbed ${\mathrm{H}}_2$, which is in agreement with theoretical calculations. Furthermore, the ${\mathrm{H}}_2$ desorption intensity ratio in $J=1$ and $J=0$ indicates fast ortho-para conversion in the adsorption state, which we estimate to be of the order of 1 s.

• Received 18 October 2017
• Revised 27 December 2017

DOI:https://doi.org/10.1103/PhysRevB.97.085436